“Regular exercise can cut your risk of colon cancer by a quarter,” reported the Daily Mail today. The newspaper said that even walking reduces the likelihood of developing one of the most common cancers, which is diagnosed in more than 35,000 people the UK.

The stories are based on a detailed analysis of 52 studies from the past 25 years. The review, which contained details from several thousand cases of colon cancer, showed that those people who were most active were 24% less likely to develop the disease than the least active individuals.

This is a high-quality review, and looked at all sorts of activity from brisk walking to jogging and heavy manual labour. It has shown that activity was linked to a reduced risk of colon cancer, and that the benefits of exercise were similar for men and women. This protection remained even after the other risk factors measured in the study, such as diet, obesity and smoking, were taken into account.

Where did the story come from?

This research was conducted by Dr Kathleen Wolin from the Department of Surgery at Washington University School of Medicine in St Louis, US, with colleagues from elsewhere in the US. Sources of funding are not reported. The study was published in the British Journal of Cancer, a peer-reviewed medical journal.

What kind of scientific study was this?

Although the link is now well-established between greater levels of physical activity and a reduced risk of colon cancer, the researchers were interested in accurately measuring the magnitude or size of the effect. To do this, they wanted to combine the results of several studies into a summary measure, a technique known as meta analysis.

The researchers restricted their analyses to case–control or cohort studies (both types of observational study) where data for colon cancer alone was available. Colon cancer occurs in the upper part of the large bowel, so the researchers excluded studies that looked at rectal (lower large bowel) cancers, or both colon and rectal cancers together, because previous studies had not shown a link between physical activity and rectal cancer. The current consensus is that such a link is unlikely to exist. Two-thirds of bowel cancer cases occur in the colon, while the rest develop in the rectum.

The researchers also say that the possibility that exercise cuts the risk of colon cancer is “plausible” because exercise is thought to reduce insulin levels in the blood, improve immune function and increase the speed at which cancer-causing chemicals in food pass through the gut.

The researchers searched the Pubmed database of research for relevant studies published up to June 2008. They searched using the terms ‘physical activity’, ‘exercise’ and ‘colon cancer’. They did not restrict studies by type of physical activity, and therefore included all studies that had measured total physical activity, recreational or leisure-time physical activity, as well as physical activity in commuting, and occupational physical activity. They excluded studies not conducted in humans, or studies that were review studies themselves. They also excluded studies where the outcome was not colon cancer or where the data was insufficient.

The researcher then recorded all the details about each study, and assessed the quality of individual research. For example, they looked at how well each study had measured physical activity and rate of patient loss in the follow-up phase of the study. They combined the results in a type of meta analysis called a random effects model. This model allows for some differences (heterogeneity) between the studies. They performed the statistical analysis and reported the results as a relative risk.

What were the results of the study?

The researchers initially identified 507 potentially relevant studies in their initial search. After exclusions, they were left with 60 studies. Of these, 52 looked at colon cancer separately from colorectal cancer and reported their results in ways that could be analysed. These suitable studies comprised of 24 case-control studies and 28 cohort studies.

In their meta analysis they found a statistically significant 24% reduced risk of colon cancer when comparing the most active individuals with the least active individuals across all studies (relative risk [RR] 0.76, 95% confidence interval 0.72 to 0.81). When they took into account (adjusted for) the quality scores for each study, the results were similar.

Separately analysing the case-control studies and cohort studies showed that the magnitude of the effect was larger for case–control (RR 0.69, 95% CI 0.65 to 0.74) compared with the cohort studies (RR 0.83, 95% CI 0.78 to 0.88). The magnitude of effect was similar in men and women (for men RR 0.76, 95% CI 0.71 to 0.82; for women RR 0.79, 95% CI 0.71 to 0.88).

What interpretations did the researchers draw from these results?

The researchers say that previous reviews of the association between physical activity and colon cancer have reported a risk reduction of approximately 30%, and that their formal meta analysis supports this. It showed a 24% risk reduction overall and similar risk reductions when men and women were examined separately.

The researchers go on to call for additional research into which types, intensities, and durations of physical activity are best, saying that this will inform public health recommendations.

What does the NHS Knowledge Service make of this study?

This is an important and well-conducted study, and its method of combining results used gave greater confidence in the reliability of the results. There are a few limitations with meta analysis of observational data, which the researchers acknowledge.

One such limitation is that different studies classified activity in a variety of ways, with some including manual work, while others concentrated on leisure, exercise such as going to the gym or running. Combining the results of these studies adds to ‘heterogeneity’ or variations in the design and methods of between studies.

Analysing heterogeneous data can be difficult, and the researchers applied a random effects model to compensate. In their analysis, the researchers also took into account the variations in time of exposure assessment, method of exposure assessment, length of follow-up, type of physical activity assessed, levels of physical activity and other possible confounders.

The researchers also separately analysed different types of study, in order to evaluate the levels of benefit seen through different research methods. The cohort studies, which are usually considered better quality evidence and less prone to bias, showed that physical activity had less of an effect than that shown in the case-control studies. This may mean that the true effect is likely to be closer to the result of the analysis of cohort studies. This may be one reason why the estimated risk reduction was 24% rather than 30%, which other studies have reported.

While the newspapers report that less intensive exercise, such as brisk walking for five to six hours a week, may be beneficial, this meta analysis cannot directly support this claim because the research did not focus on specific types or intensities of exercise.

While exercise has many benefits, the researchers are sensible to call for more research to determine the type, intensity and duration of exercise needed to reduce colon cancer risk. In turn, this further research could form the basis for more detailed public advice on how intensive and how long this physical activity needs to be.

However, in the meantime it appears that some exercise is better than no exercise, and it can prevent an expanding range of conditions. Even light exercise, such as walking, can benefit heart health, so it is sensible to incorporate it into daily life where possible.